Callery pears: An invader ‘worse than murder hornets!’
Stinky but handsome and widely popular landscape trees have spawned aggressive invaders, creating thickets that overwhelm native plants and sport nasty four-inch spikes.
Bradford pears and 24 other ornamental trees were developed from Callery pears — a species brought to America a century ago to save ravaged pear orchards. Now, their invasive descendants have been reported in more than 30 states.
“Worse than murder hornets!” was the tongue-incheek title of a U.S. Department of Agriculture webinar in 2020 about Callery pears including the two dozen thornless ornamental varieties sold since the 1960s.
“They’re a real menace,” said Jerrod Carlisle, who discovered that four trees in his yard and one at a neighbor’s had spawned thousands on 50 acres (20 hectares) he was turning from cropland to woods in Otwell, a community of about 400 in southern Indiana.
Indiana is among 12 Midwestern and western states that have reported invasions, though most are in the South and Northeast.
Until 2015, Carlisle rented his field to a farmer. Then he enrolled it in a USDA crop reduction program that paid for planting 29,000 trees as wildlife habitat.
Carlisle realized the spiky flowering pears were a problem in 2019. When he cut or mowed them, new sprouts popped up. Trees sprayed with herbicide regrew leaves. Cutting off bark in a circle around the trunk kills most trees. Not these.
He and his 17-year-old son have cut down an estimated 1,400 Callery pears, applying herbicide to the stumps. But he figures there are about 1,000 more to go.
Without regular maintenance, fields near seed-producing trees can be covered with sprouts within a couple of years, said James “J.T.” Vogt, a scientist at the U.S. Forest Service’s Southern Research Station in Athens, Georgia.
“If you mow it, it sprouts and you get a thicket,” he said. “If you burn it, it sprouts, too.”
Seedlings only a few months old bear spurs that can punch through tractor tires, said David R. Coyle, an assistant professor in Clemson University’s Department of Forestry and Environmental Conservation.
The stench wafting from the tree’s billows of white blossoms has been compared to perfume gone wrong, rotting fish, chlorine, and a cheese sandwich left in a car for a week. The trunks branch off in deep Vs, so after 15 to 20 years they tend to break in storms.
But Frank N. Meyer, an agricultural explorer who brought 2,500 species of plants including his namesake Meyer lemon to the USDA in the early 1900s, called the Callery pear wonderful, noting that it survived drought and poor soil.
At the time, a bacterial disease called fire blight was devastating U.S. pear orchards, University of Cincinnati researchers Theresa M. Culley and Nicole A. Hardiman wrote in a 2007 BioScience article about the plant’s U.S. history.
And, just as researchers had hoped, grafting edible pears onto Callery roots produced blight-resistant fruit trees.
In 1952, USDA workers noticed a spikeless mutant growing among Callery pears started from seed. By grafting its cuttings onto roots of other Callery pears, they cloned an ornamental line they named Bradford pears. That variety was commercially available by 1962, Culley and Hardiman wrote.
Other seedlings grew into 24 more ornamental varieties. All are so pretty, hardy and insect-resistant that they were planted nationwide.